1. Field of the Invention
The present invention relates to material handling vehicles, such as lift trucks and reach trucks, that have electrically operated components and a hydraulic system; and more particularly to techniques for recovering energy from the hydraulic system to power the electrically operated components.
2. Description of the Related Art
Certain material handling vehicles have electric motors for driving and steering the wheels that propel the vehicle across the floor of a manufacturing plant, a warehouse or similar facility. These material handling vehicles also have a hydraulic system for raising and lowering a load carrier, such as a pair of forks or a platform. The hydraulic system has an electric motor that operates a pump which supplies pressurized fluid. The pressurized fluid is controlled by a valve assembly to operate a hydraulic cylinder and ram assembly to raise the load carrier.
The load carrier is lowered by the force of gravity. The rate of lowering is controlled by opening a valve in a manner that proportionally controls the flow of fluid out of the cylinder to a reservoir. The fluid flows out of the cylinder under pressure due to the force of gravity acting on the load carrier and any load thereon. That pressure is reduced to the atmospheric level upon the fluid entering the reservoir. Therefore the energy contained in that exhausting pressurized fluid is lost.
Some material handling vehicles have a mechanism for recovering the energy in that exhausting fluid. Instead of routing that fluid directly to the reservoir, the fluid is directed backwards through the pump to the reservoir, thereby driving the pump as a hydraulic motor. Driving the pump in that manner also drives the electric motor coupled to the pump causing the electric motor to act as a generator. The resulting electricity that is generated is either used to power other active devices on the material handling vehicle or used to recharge a vehicle battery.
A drawback of this energy recovery technique occurs at flow rates of the fluid exhausting from the hydraulic cylinder. Excessively high flow can drive the pump and electric motor at too great a speed resulting in excessive wear on both devices. Very high speed also generates electric frequencies that may adversely affect the excitation system on the vehicle.
Therefore, there is a desire to be able to recover energy from the exhausting fluid at high flow rates without adversely affecting the pump-motor combination.